1. Field of the Invention
The present invention generally relates to an engine equipped with two throttle valves along its air intake passage. More particularly, this invention relates to an engine control apparatus, which regulates one throttle valve to permit a vehicle to move at constant speed, even when the driver is not stepping on the accelerator pedal, and which regulate the other throttle valve in order to control the engine power.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 1-167429 discloses an engine having two throttle valves disposed along its air intake passage. The angle of the first throttle valve is mechanically controlled in relation to the amount of manipulation or thrusting of the accelerator pedal. The angle of the second throttle valve is controlled in relation to the amount of manipulation of the actuator. The conventional control apparatus disclosed in this publication adjusts the angle of the second throttle valve, in order to control the engine power. The control apparatus has characteristic control maps of different patterns to determine the second throttle valve angle as a function of the first throttle valve angle.
When the slip ratio, which is computed based on the numbers of rotations of the drive wheel and driven wheel, exceeds a predetermined value, or falls below it, the control apparatus refers to the engine power control maps to reduce the second throttle valve angle. As a result, the opening of the air intake passage at the second throttle valve is reduced, to prevent the wheels from slipping. Thereafter, the control apparatus sequentially changes control maps, in order to gradually to set the second throttle valve angle back to the original angle.
Japanese Unexamined Patent Publication No. 2-124329 also discloses an engine having two throttle valves disposed along its air intake passage. In this conventional embodiment, the first throttle valve is driven by the actuator as well as by the accelerator pedal. The control of the constant cruising speed of a vehicle is performed by controlling the first throttle valve angle, while the engine power is controlled by adjusting the second throttle valve angle.
However, the conventional control devices which select one of the different control modes (characteristic control maps), based on the second throttle valve angle that is associated with the first throttle valve angle, in order to control the second throttle valve, have the following shortcomings. If the first throttle valve is regulated to maintain the constant cruising speed of the vehicle, sufficient engine torque might not be reached at the time the vehicle is set in the constant cruising speed mode, thus causing the vehicle speed to exceed or not to reach the target value. This phenomenon will be referred to as "overshooting or under shooting the target valve. There are two possible causes for this phenomenon. One cause is that the first throttle valve angle that is required to maintain a specific constant speed varies with the different control modes. Another cause is that the conventional control devices do not take into consideration the control mode of the second throttle valve, in setting the initial angle of the first throttle valve, at the onset of the constant cruising speed mode.
The overshooting and undershooting phenomenon will be described in more detail referring to FIGS. 2 through 4.
The method for determining the angle of the second throttle valve will be explained using the graph in FIG. 2. FIG. 2 shows a two-coordinate system, where the abscissa represents the angle of the first throttle valve, and the ordinate represents the total equivalent throttle angle. The total equivalent throttle angle means the equivalent valve angle when the first and second throttle valves are replaced with a single throttle valve. Normally, the equivalent valve angle is closer to the smaller one of the two valve angles. For instance, when the first throttle valve angle is 20.degree. and the second throttle valve angle is 50.degree., the total throttle equivalent angle is approximately 20.degree..
In the control mode (A) illustrated by the broken line in FIG. 2, the second throttle valve will have a smaller angle than that of the first throttle valve. In the control mode (B) illustrated by the solid line in FIG. 2, the second throttle valve angle is nearly proportional to the angle of the first throttle valve. Even if a certain total equivalent throttle angle were sought, the required angle of the first throttle valve varies with the control modes. For example, when the total equivalent throttle angle of x.degree. is sought, the required first throttle valve angle is a.degree. in the control mode (A), and b.degree. in the control mode (B).
The relationship between the first throttle valve angle and the engine torque in individual control modes will now be explained using the graph in FIG. 3. FIG. 3 shows a two-dimensional coordinate system, where the abscissa represents the angle of the first throttle valve, and the ordinate represents the engine torque. The broken line in FIG. 3 represents the control mode (A) for the second throttle valve, and the solid line represents the control mode (B). If a certain engine torque were sought, the required first throttle valve angle varies with the control modes. For example, when the engine torque of "Q" is sought, the required first throttle valve angle is a.degree. in control mode (A), and b.degree. in control mode (B). The graph illustrates that the engine torque for the given first throttle valve angle differs with the control modes. For example, with the first throttle valve angle of x.degree., the engine torque is Qa in the control mode (A), and Qb (Qa&lt;Qb) in the control mode (B).
A change in the vehicle speed at the time the constant cruising speed starts, will be described referring to FIGS. 4A through 4D. Each diagram includes a graph showing the relationship between time and the vehicle speed, and a graph showing the relationship between time and the manipulation amount of the actuator for driving the first throttle valve. The actuator manipulation amount means the angle by which the actuator rotates to control the first throttle valve angle to a predetermined valve. The actuator manipulation amounts A.degree. and B.degree. shown in these four diagrams respectively correspond to the first throttle valve angles a.degree. and b.degree. in FIG. 2.
In FIGS. 4A and 4B, the actuator manipulation amount for use in constant cruising speed is determined on the premise that the second throttle valve is always regulated in the control mode (A). In FIGS. 4C and 4D, the actuator manipulation amount is determined on the premise that the second throttle valve is always regulated in the control mode (B). In those four figures, it is assumed that the target value of the vehicle speed in the constant cruising speed is the same, and that the engine torque needed to keep the target vehicle speed is "Q", as indicated in FIG. 3.
FIG. 4B illustrates the case where the actuator manipulation amount (or the first throttle valve angle) is set on the premise that the second throttle valve is regulated in the control mode (A), and where the second throttle valve is actually regulated in this control mode (A). When the constant cruising speed is set at time t1, the control apparatus considers the vehicle speed at that time, as the target vehicle speed for the constant cruising speed. Further, the control apparatus sets the actuator manipulation amount to A.degree., in order to provide the first throttle valve angle a.degree. needed to acquire a predetermined engine torque (Q) in the control mode (A). In this case, as the engine torque becomes "Q", no overshooting or undershooting of the vehicle speed occurs, and the real vehicle speed increases or decreases so as to stabilize at the target vehicle speed.
FIG. 4A illustrates the case where the actuator manipulation amount is set on the premise that the second throttle valve is regulated in the control mode (A), but the second throttle valve is actually controlled in control mode (B). After the constant cruising speed is set at time t1, the vehicle speed overshoots the target speed.
The conventional control devices do not account for the control mode of the second throttle valve, for controlling the constant cruising speed. Therefore, when the constant cruising speed is set at time t1, the initial actuator manipulation amount is set to A.degree., in order to set the engine torque to "Q", although the second throttle valve is actually controlled in the control mode (B). However, as the second throttle valve is actually controlled in the control mode (B), the engine torque becomes Qa', such that Qa' is greater than Q, as shown in FIG. 3. Thus, the real vehicle speed overshoots the target vehicle speed, and the actuator manipulation amount gradually decreases toward B.degree. through a conventional constant cruising speed feedback system. As shown in FIG. 3, the actuator manipulation amount B.degree. corresponds to the torque Q.
FIG. 4D illustrates the case where the actuator manipulation amount (or the first throttle valve angle) is set on the premise that the second throttle valve is regulated in the control mode (B), and where the second throttle valve is actually regulated in this control mode (B). When the constant cruising speed is set at time t1, the control apparatus considers the vehicle speed at that time as the target vehicle speed for the constant cruising speed. Further, the control apparatus sets the actuator manipulation amount to B.degree., to provide the first throttle valve angle b.degree. that is needed for the engine to generate a predetermined engine torque (Q) in the control mode (B). In this case, as the engine torque actually becomes "Q", the real vehicle speed reaches the target vehicle speed, without overshooting or undershooting it.
FIG. 4C illustrates the case where the actuator manipulation amount is set on the premise that the second throttle valve is regulated in the control mode (B), but where the second throttle valve is actually regulated in the control mode (A). In this case, the vehicle speed undershoots the target speed, after the constant cruising speed is set at time t1.
As mentioned earlier, the conventional control devices do not take into consideration the control mode for the second throttle valve when controlling the constant cruising speed. Therefore, when the constant cruising speed is set at time t1, the initial actuator manipulation amount is set to B.degree. to set the engine torque to "Q", although the second throttle valve is actually regulated in the control mode (A). However, the second throttle valve is actually regulated in the control mode (A), and, as shown in FIG. 3, the engine torque becomes Qb', which is smaller than Q. Thus, the real vehicle speed undershoots the target vehicle speed, and, as shown in FIG. 4C, the actuator manipulation amount gradually decreases after temporarily exceeding the predetermined valve, and converges toward A.degree. through a conventional constant cruising speed feedback system. The actuator manipulation amount A.degree. corresponds to the torque Q.
Briefly, the conventional methods do not attach significant importance to the control mode for the second throttle valve in regulating the constant cruising speed of the vehicle. In other words, the initial actuator manipulation amount (i.e., the initial first throttle valve angle) will always be accorded exclusive or higher priority in association with the selected control mode. When the actual initial manipulation amount differs from the initial manipulation amount, which corresponds to the control mode that is used in the constant cruising speed, the vehicle speed could overshoot or undershoot the target speed.